Data collected in the CPP are being analysed to determine the primary and contributing roles of biological, environmental and familial factors in mental retardation in a biracial population of 40,000 seven year old children followed since the prenatal period. In this population, approximately two percent of the white sample and five percent of the Negro sample have IQs under 70. Preliminary findings indicate that the following conditions are related to a low level of intellectual functioning at age seven: Low socioeconomic status of the family which includes income, and occupation and education of the parents, high housing density, low maternal intelligence test scores, less prenatal care, more complications in the neonatal period, less advanced mental and motor development at 8 months, more frequent signs of central nervous system involvement at one year, poorer language development at age three, lower psychometric test performance at age four, and smaller physical measurements during the first 7 years of life.